1. Field of the Invention
The present invention relates to a heat shrinkable film, in particular heat shrinkable multilayered film, utilizing an ethylene-vinyl alcohol copolymer (hereinafter referred to as "EVOH") composition having excellent heat stretchability, in particular that under low tension, that can be stretched while forming no pinholes, cracks, local non-uniform thicknesses or like defects, and having excellent heat shrinkability and gas barrier properties.
2. Description of the Prior Art
Fatty foods such as raw meat and processed meat mostly have irregular shapes and non-uniform sizes. To wrap these foods having various shapes, shrink-wrapping using heat shrinkable films and skin-pack system that carries out stretching, shrinkage and heat sealing at the same time through a skin-packer are commercially practiced.
The shrink-wrapping process generally comprises the successive steps of placing the contents in a tubular stretched film, removing the air inside the tube under reduced pressure and sealing the openings of the tube and shrinking the film to permit it to conform to the shape of the contents closely, thereby obtaining a neat package. The heating in this process is-conducted, sometimes, also to sterilize the contents and generally at a temperature of 70.degree. to 120.degree. C.
The skin-pack process generally comprises the successive steps of passing an unoriented film through a skin-packer, heating the film in a heating zone to 60.degree. to 200.degree. C., in some cases thermoforming the film in a mold, immediately thereafter laying it, in vacuo, on the contents placed on a substrate film, sheet or tray, sealing the periphery, restoring the pressure to atmospheric pressure to permit the film to conform to the shape of the contents, thereby obtaining a neat package.
It is necessary that films usable in these processes have excellent gas barrier properties and the good property of conforming closely to the shape of the contents and be readily stretchable at 40.degree. to 200.degree. C. and, at the same time, sufficiently shrinkable by heating.
One may consider use of EVOH film having high gas barrier properties as the above stretchable-shrinkable film. The EVOH film having high gas barrier properties however has the disadvantage of poor stretchability-shrinkability.
Various processes have been proposed to solve this problem. For example, Japanese Patent Application Laid-open No. 115880/1977 proposed a composite film obtained by laminating EVOH film and a polyamide (PA) film and stretching the laminate, and Japanese Patent Application Laid-open No. 136365/1981 a multilayered film comprising a polyolefin layer, a PA layer and an EVOH layer.
Japanese Patent Application Laid-open No. 261847/1992 discloses a process which comprises heat stretching (e.g. heat biaxially stretching) an EVOH composition having an ethylene content of 20 to 60 mol % and at least 2 endothermic peaks upon DSC measurement to obtain multilayered articles (films, sheets, containers or the like), in particular thermoformed containers, that have no pinholes, cracks or local non-uniform thicknesses and excellent gas barrier properties. The document however does not describe provision of any heat shrinkable film or manufacturing conditions therefor.
U.S. Pat. No. 4,590,131 (Japanese Patent Application Laid-open No. 173038/1985) discloses a packaging material (film, sheet, container or the like) having excellent stretchability and comprising a composition of a plurality of EVOH's having different ethylene contents. Example 5 of the document describes that an inside bag with no cracks and usable for bag-in-box was obtained by co-extrusion molding by tubular film process a multilayered film comprising an intermediate layer of a blend of EVOH's and inner and outer layers of a linear low density polyethylene (LLDPE). The Example however does not describe obtaining a heat shrinkable film or manufacturing conditions therefor.
Japanese Patent Application Laid-open Nos. 192705/1985, 199004/1985 and 4752/1986 describe that a composition of EVOH's having different ethylene contents has improved stretchability, but do not describe obtaining any heat shrinkable film or manufacturing conditions therefor.
U.S. Pat. No. 4,349,644 (Japanese Patent Application Laid-open No. 86949/1981) describes that a composition of EVOH's having different ethylene contents has improved melt processability, but describes nothing about obtaining a heat shrinkable film or manufacturing conditions therefor.
Lamination of EVOH resin alone with a PA resin does not always improves stretchability, in particular that under low tension, and it can hardly be said that this method gives a film having excellent heat shrinkability.
There is still desired a film having high gas barrier properties and, at the same time, having excellent stretchability, heat shrinkability and dimensional stability.